This invention relates to microporous materials or films comprising a compatible, melt-processable polymer mixture and a method of making the same. The material or film is useful as a single layer or as part of a multi-layer structure and is useful in articles such as battery separators.
Porous films made from mixtures of blends of polyolefin materials used as membranes and as thermal shutdown separators for batteries are known. Shutdown separators are safety devices which, for example in the case of lithium ion batteries, prevent dangerous overheating by closing off the pores (and effectively stopping the electrolytic reaction) at a temperature reached before a dangerous condition exists. They effectively act as fuses.
Briefly, the present invention provides a microporous material comprising a polypropylene polymer having at least 20 percent crystallinity; and a compatible, amorphous, glassy polymer, wherein said polymers are miscible in a compound when heated above the melting temperature of the polypropylene polymer and phase separate from the compound when cooled.
In another aspect, the present invention provides a method of making microporous material, comprising: (a) melt blending to form a mixture comprising about 15 to 80 parts by weight of a polypropylene polymer having at least 20 percent crystallinity, greater than 0 to 30 parts by weight of a compatible, amorphous, glassy polymer, and about 20 to 85 parts by weight, based on a total mixture content, of a compound, the compound being miscible with the polypropylene and glassy polymer at a temperature above the melting temperature of the polypropylene polymer; (b) shaping the melt blended material; (c) cooling the shaped material to a temperature at which phase separation occurs between the compound and the polymers through crystallization of the polypropylene polymer to form a network of polymer domains; and (d) creating porosity by at least (1) orienting said material at least in one direction to separate adjacent crystallized polymer domains from one another to provide an interconnected porous network therebetween, or (2) removing at least a part of the compound. When creating the porosity, both steps of orienting the material and removing at least a part of the compound may be combined.
In still another aspect, the present invention provides a method of making microporous material, comprising (a) melt blending to form a substantially homogeneous mixture comprising about 15 to 80 parts by weight of a polypropylene polymer having at least 20 percent crystallinity, greater than 0 to 30 parts by weight of a compatible, amorphous, glassy polymer, and about 20 to 85 parts by weight, based on a total mixture content, of a compound, the compound being miscible with the polypropylene and glassy polymer at a temperature above the melting temperature of the polypropylene polymer; (b) shaping the melt blended material; (c) cooling the shaped material to a temperature at which phase separation occurs between the compound and the polymers through crystallization of the polypropylene polymer to form a network of polymer domains; (d) stretching the shaped material in at least two perpendicular directions to provide a network of interconnected pores; and (e) removing the compound to provide a microporous material. The step of stretching the material may be performed before the step of removing the compound.
As used herein xe2x80x9ccompatiblexe2x80x9d with reference to polymers means one material able to form a substantially homogeneous mixture with a polymer with which it is compatible, said mixture capable of forming a transparent cast or extruded film. Such a material can be compatible with such a polymer within a certain weight ratio range of material to polymer and incompatible (i.e., unable to form a transparent film) outside that weight ratio range.
xe2x80x9cMisciblexe2x80x9d means a solute (polymer) is soluble in a solvent (the compound) such that the combination of solute and solvent appears to be a single phase exhibiting no apparent phase separation, which may depend on the temperature of the combination.
It is an advantage of the present invention to provide a porous matrix resistant to pore collapse arising from capillary forces that occur during drying. It is another advantage of the present invention to provide a microporous material with improved thermal stability.